Elucidation of how the size of a developing embryo is genetically regulated is important because the final volume of endosperm as a storage organ of starch and proteins is affected by embryo size in cereal crops. Researchers have found that embryo size-related genes contribute to the regulation of endosperm development. Investigation of these genes is important for agriculture because cereal endosperms are the staple diet in many countries. Also, it is important for agriculture because embryos of various crop grains are the source of many valuable nutrients including oil.
The giant embryo (ge) mutation was first described by Satoh and Omura (1981) Jap. J. Breed. 31:316-326. The giant embryo mutant is a potentially useful character for quality improvement in cereals because increased embryo size will result in increased embryo oil and nutrient traits that are desirable for human consumption. Also, the enlargement of embryos would result in increased embryo-related enzymatic activities, which are often important features in the processing of grains. The mutation was genetically mapped to chromosome 7 (Iwata and Omura (1984) Japan. J. Genet. 59: 199-204; Satoh and Iwata (1990) Japan. J. Breed. 40 (Suppl. 2): 268-269), with additional ge alleles also localized to chromosome 7 (Koh et al. (1996) Theor. Appl. Genet. 93:257-261). The ge mutations were analyzed at the morphologic and genetic level by Hong et al. (1994) Development 122:2051-2058. This publication linked the GE gene as being required for proper endosperm development. Since both endosperm and embryo size are affected by the mutation, GE appears to control coordinated proliferation of the endosperm and embryo during development. Beside the morphological change of embryo and endosperm in ge, it was also shown that the ge seed accumulates more oil compared to the wild type (Matsuo et al. (1987) Japan. J. Breed. 37: 185-191; Okuno (1997) In “Science of the Rice Plant” Vol. III, Matsuo et al. eds., Food and agriculture policy research center, Tokyo, Japan, pp 433-435).
It has been found that loss-of-function of the GE gene leads to an enlargement of embryonic tissue at the expense of endosperm tissue. This developmental change may be useful in increasing the amount of embryo-specific metabolites such as oil in seed-bearing plants. Despite the extensive genetic and morphological characterization of the GE gene there has been no molecular analysis of the nucleic acid encoding this protein. Indeed, the identity of the protein encoded by GE has not been reported. A better understanding of the GE gene, and the protein it encodes, will be required for a complete understanding of the process controlling embryo size in rice.